Non-harmonic vibrato devices are known, typified by U.S. Pat. No. 2,741,146, which allows the musician to change the tension on all guitar stings in unison by activating a lever, without correcting for relative pitch between strings.
Subsequent devices, typified by Jones, U.S. Pat. No. 3,411,394, correct pitch by varying the length of a crank arm or the radius of a string bearing cam. These devices suffer from one or more of the shortcomings of imprecise geometry, expressive difficulty, lack of range, tuning difficulty, tuning instability.
Methods previously used to stabilize a vibrato, such as cam locks, or flats on activating cams, interfere with the smooth expressive motion of the vibrato.